A major problem of the calenders used, e.g. in the plastic industry for the manufacture of thin plastic foils or films, has been the deflection of the rolls, especially the long rolls, which deflection varies at adjacent rolls and thus gives rise to variations in the roll nip in a direction transverse to the roll assembly. This naturally also gives rise to irregularities and variations in the foil profile in a direction transverse to the manufactured web, above all variations in the thickness. The negative effect of the uneven profile is especially apparent in thin plastic webs. An uneven foil is also difficult to use.
Different means have been used in an attempt to eliminate the above mentioned disadvantages caused by the roll deflection. Attempts have been made i.a. to bend the roll in the opposite direction to counteract the deflection. This is achieved by applying a load on the roll axis at both ends outside the bearing points, which load opposes the pressing force. In this way it is possible to compensate for the deflection of the adjacent roll to some degree, but not entirely, and this results in an oxbow profile of the mmaterial to be treated.
Another method of compensating consists in inclining the roll axis in relation to the adjacent roll axes, but also in this case the result is similar, that is an oxbow profile is obtained.
The most advantageous method of compensating for the roll deflection consists in crowning the rolls in accordance with the roll deflection. In this way a constant nip width is obtained, but only if all the factors on which the desired crowning is based, i.e. the working speed of the calender, the composition of the material and the temperature and material thickness etc., remain unchanged. If one of these factors is changed, also the crowning of the roll should be corrected. This is, however, very difficult to carry out in practice.
It is also known to adjust the roll thickness at certain areas along the rolls by alternatively heating or cooling the areas in question, as required.
Finally also so called "non-bending" rolls exist which are very expensive, but not even the characteristics of these rolls are sufficient to cover completely all existing fields of application.